1. Field of the Invention
The present invention relates to a method for titrating nitrate and/or nitrite ions in solution at a substantially neutral pH.
2. Description of Related Art
It applies, typically but not exclusively, to the use of a copper electrode for the potentiostatic titration of nitrate and/or nitrite ions in an aqueous medium having a substantially neutral pH.
Many methods for quantitatively determining nitrate and/or nitrite ions are known, in particular at a highly acidic pH. Mention may be made, for example, of the document entitled “The effect of surface preparation of a copper electrode on the reduction of nitrate ions” by Aljaz Coh and Boris Pihlar (Acta Chimica Slovenia, 43/1/1996, pp. 5-12). This document exhibits a method for the titration of nitrate ions using a copper electrode.
This titration method comprises a first stage which consists in applying a potential of between 0 and 0.2 V vs. Ag/AgCl to the copper electrode for a predetermined time, so as to oxidize the metal copper of said electrode and thus form a layer of copper oxide, and a second stage which consists in titrating, by reduction, the nitrate ions by scanning the potential applied to the copper electrode (“potentiodynamic mode” titration), the first and second stages being carried out in a highly acidic medium in a solution of the 0.1 M HClO4/0.9 M NaClO4 type. Furthermore, even if this document mentions that a layer of copper oxide is formed, it is difficult to envisage that the layer of copper oxide can exist in a highly acidic medium.
The plot of the curve of the intensity as a function of the potential applied to the copper electrode then makes it possible to identify the current peak due to the reduction of nitrate ions to give NH4+ ions and to thus deduce therefrom the concentration of nitrate ions by calibration in the acidified aqueous medium.
In addition, according to this method, it is preferable to add Cu2+ ions to the acidic medium to be analyzed in order to catalyze the reduction of the nitrate ions during the titration stage.
However, during the implementation of this titration method in potentiodynamic mode, that is to say with gradual scanning of the potential applied to the copper electrode, a gradual decrease in the current measured is observed, which decrease is due to poisoning of the electrode, and it is thus impossible to carry out reproducible measurements one after the other.
Furthermore, the concentration of nitrate ions cannot be measured in a technically simple way since the variation in the potential with a given rate, in potentiodynamic mode, involves the use of relatively complex electronic equipment.
In addition, by this method, it is not possible to titrate the nitrate ions in an unmodified natural medium as it is necessary to add a strong acid, indeed even Cu2+ ions, to the medium to be analyzed.
Finally, the measurements in an acidic medium are limited since they do not make it possible to observe the reduction of the nitrite ions. This is because, when the medium is acidified, the nitrite ions initially present in the medium are in the form of nitrous acid (HNO2, pKa=3.3). Nitrous acid is unstable and then disproportionates to give nitrates and NO.